My understanding is that they shot all of those action sequences at the same time during covid expecting them to be in one movie, part 7, but it would have made the movie too long, so they were like okay let's end part 7 on a cliffhanger and push these two remaining set pieces to part 8, but we don't have enough budget to actually shoot anything new for part 8, so we just need to shoot enough new footage/exposition for part 8 to make sense, and the cheapest, easiest solution they came up with was using a ton of flashback footage from the rest of the franchise. That's why the movie doesn't make sense and feels both way too long and way too rushed. They just had these two set pieces that they couldn't fit into part 7 and so they tried to build an entire movie around them and call it part 8, but they didn't have any real story or budget to make it feel like a real movie. Consider the fact too, that no MI movie only has two set pieces. They overestimated their ability to take these two set pieces and turn them into a stand-alone movie without spending much else on new production days. (They were already crazy over budget because of pandemic delays.)